1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device capable of improving picture quality by preventing light leakage by arranging a data dummy line, and a method of fabricating the same.
2. Discussion of the Related Art
Recently as information has become more available, displays have become more and more important as a visual information presentation media. Going forward to the future, displays need to have low power consumption, be thin and light, have a high picture quality, etc. Liquid crystal display (LCD) devices are equipped with not only functions to satisfy these conditions but are also easily mass produced, so that each kind of new LCD device may be quickly commercialized resulting in LCD devices replacing cathode ray tubes (CRT).
A general LCD device displays an image by controlling an optical transmittance ratio of a liquid crystal using an electric field. To this end, the LCD device is composed of a color filter substrate, an array substrate, and a liquid crystal material layer formed between the color filter substrate and the array substrate.
FIG. 1 is a plan view showing a part of the array substrate of a general LCD device.
As shown in FIG. 1, the array substrate 10 includes a gate line 16n and a data line 17 defining a pixel region by being arranged horizontally and vertically, a thin film transistor (TFT) 20 located at the crossing region between the gate line 16n and the data line 17 and acts as a switching device, and a pixel electrode 18 formed at each pixel region.
The TFT 20 includes of a gate electrode 21 connected to the gate line 16n, a source electrode 22 connected to the data line 17, and a drain electrode 23 connected to the pixel electrode 18. In addition, the TFT 20 includes a first insulating layer (not shown) for insulating the gate electrode 21 and the source/drain electrodes 22 and 23, and a semiconductor layer (not shown) for forming a conductive channel between the source electrode 22 and the drain electrode 23 by a gate voltage supplied to the gate electrode 21.
The drain electrode 23 is electrically connected to the pixel electrode 18 of the pixel region through a first contact hole 24 formed in a second insulating layer (not shown).
Generally, the pixel electrode of the array substrate forms a liquid crystal capacitor with a common electrode of a color filter substrate. A voltage applied to the liquid crystal capacitor is not maintained until the next signal is introduced, but instead leaks, thus disappearing. Therefore, in order to maintain the applied voltage, a storage capacitor has to be connected to the liquid crystal capacitor.
The storage capacitor not only maintains the signal voltage but also stabilizes the gray scale display and reduces flicker and a residual image.
A parasitic capacitance (Cgs) results in an overlapped area between the gate electrode and the source/drain electrode. The pixel voltage is varied as much as V due to the parasitic capacitance, which is called a level shift voltage or a kickback voltage and can be expressed as the following formula 1.V=Cgs/(Cgs+Clc+Cst)×Vg  [Formula 1]
Herein, Vg denotes Vghigh−Vglow, Cls denotes a capacitance of a liquid crystal capacitor, and Cst denotes a capacitance of a storage capacitor.
A characteristic of the liquid crystal varies when a direct current voltage is applied to the liquid crystal in one direction for a long time. Therefore, a polarity of an applied voltage has to be periodically changed at the time of driving liquid crystal. A direct current component due to an asymmetrical structure of a positive polarity (+) and a negative polarity (−) remains as indicated by the V, thereby causing a flicker of a screen, a residual image, an uneven brightness, etc.
According to this, by introducing an storage capacity Cst, the size of the V is reduced thus to improve a picture quality.
The storage capacitor can be formed by two methods. First, an electrode for a storage capacitor is additionally formed and is connected to a common electrode thus to be used as the storage capacitor. Second, a part of the n−1th gate line is used as an electrode of a storage capacitor of the nth pixel. Herein, the former is called as a storage on common (SOC) method or an independent storage capacitor method, and the latter is called as a storage on gate (SOG) or a previous gate method.
In the LCD device shown in FIG. 1, the storage capacitor is formed by using a storage on gate method. That is, the pixel electrode 18 of the nth pixel and a part of the previous gate line 16n-1 which is the n−1th gate line (that is, a first storage electrode) are overlapped thus to form a storage capacitor 30 with the first insulating layer in between. The pixel electrode 18 of the nth pixel is connected to a second storage electrode 33 through a second contact hole 34 formed at the second insulating layer, thereby forming the first storage electrode and the storage capacitor.
As the LCD device becomes larger, the capacitance of the storage capacitor has to increase in order to stably maintain the pixel voltage. However, using the aforementioned LCD device, increasing the capacitance of the storage capacitor is limited.
For a passivation layer of the array substrate, an inorganic insulating layer having a great dielectric constant such as SiNx or a SiO2 is used. The pixel electrode and the data line having the inorganic insulating layer therebetween have to maintain a horizontal interval of 3˜5 μm in order to minimize a coupling effect due to the parasitic capacitor. As a result, light leaks through a gap between the data line and the pixel electrode thus lowering the picture quality of the LCD device.
In order to shield light that causes picture quality degradation, the width of a black matrix formed on the color filter substrate has to be increased to shield the light leakage region. However, when the pixel pitch is increased and a large substrate is used, the width of the black matrix has to be designed with greater consideration of deviations during the attachment process. As a result, the aperture ratio of the LCD device decreases.